1. Field of the Invention
The present invention relates to wireless communication devices and methods of restraining dispersion of a propagation environment index. More particularly, the present invention relates to a wireless communication device for performing wireless communication such as W-CDMA (Wideband-Code Division Multiple Access) communication and a method of restraining dispersion of a propagation environment index used in such wireless communication.
2. Description of the Related Art
In recent years, what is called HSDPA (High Speed Downlink Packet Access), which is a wireless communication scheme based on the W-CDMA technology, has been actively researched and developed. HSDPA provides high-speed downlink packet transmission at a maximum of 14.4 Mbps (on average, 2 to 3 Mbps), which is three to four times faster than the downlink transmission rate of existing W-CDMA systems, and is standardized by the 3GPP Release 5 (3rd Generation Partnership Project Release 5).
FIG. 11 illustrates an overview of HSDPA. Mobile phones 111 to 113 and notebook computers 114 and 115 exist in the cell 100a of a base station 100. As illustrated, downlink packet transmission from the base station 100 to the mobile phone 111 and the notebook computer 114 is carried out according to the conventional W-CDMA scheme, while downlink packet transmission from the base station 100 to the mobile phones 112 and 113 and the notebook computer 115 is performed according to the HSDPA scheme.
In W-CDMA, packets are transmitted from the base station 100 at a uniform rate (maximum: 384 Kbps) regardless of where in the cell 100a the mobile phone 111 and the notebook computer 114 are located.
On the other hand, in HSDPA, the current radio wave reception states of individual terminals are detected and modulation schemes are switched so that the fastest modulation scheme may be selected. Accordingly, even though the terminals are located in the same cell 100a, available downlink transmission rates differ depending on the receiving conditions such as the distance from the base station.
For example, if the mobile phone 112 and the notebook computer 115 are located near the base station 100 and are in good receiving conditions without any obstacle therebetween, the mobile phone 112 and the notebook computer 115 can receive data at a maximum rate of 14.4 Mbps. On the other hand, if the mobile phone 113 is located near the border of the cell 100a far away from the base station 100 and is in bad receiving conditions, the mobile phone 113 receives data at a lower rate than 14.4 Mbps.
Thus, in HSDPA, adaptive modulation and coding process is carried out in accordance with the receiving conditions such that the downlink transmission rate is optimized. Specifically, the modulation scheme is switched between QPSK (Quadrature Phase Shift Keying: a modulation scheme in which only the phase of the carrier wave is changed to four states to transmit 2-bit information per symbol) used in the existing W-CDMA systems and 16 QAM (Quadrature Amplitude Modulation: a modulation scheme in which the phase and amplitude of the carrier wave are changed to create 16 states, thereby transmitting 4-bit information per symbol).
In addition to the switching of modulation schemes, a channel format (number of codes allocated, etc.) for transferring data to mobile terminals is also adaptively set in accordance with the receiving conditions. Thus, HSDPA permits high-speed downlink packet transmission as stated above and is expected to become a promising technique enabling high-speed mobile communication services.
As conventional techniques for HSDPA, there has been proposed a technique in which a temporal fluctuation characteristic of the reception quality of a mobile station is estimated and a target error rate is switched in accordance with the estimated temporal fluctuation characteristic so that the highest possible throughput may be obtained (e.g., Japanese Unexamined Patent Publication No. 2005-86304 (paragraph nos. [0025] to [0029], FIG. 1)).
Meanwhile, a base station for HSDPA carries out scheduling in accordance with radio wave receiving conditions, to select users to whom information is to be preferentially transmitted or to switch the modulation scheme or the channel format.
To perform the scheduling, the base station sends out a pilot signal with a certain carrier frequency, and mobile terminals present in the cell, such as mobile phones, receive the pilot signal.
When the pilot signal is received, each mobile terminal measures the current propagation environment and notifies the base station of a propagation environment index corresponding to the measured propagation environment. The base station transmits traffic data preferentially to mobile terminals in good propagation environments or selects suitable modulation schemes or channel formats.
Specifically, the propagation environment index denotes a CQI (Channel Quality Indicator) which is obtained by converting the SIR (Signal-to-Interference Ratio) of the pilot signal to one of 30 index values from “1” to “30” (indicative of the receive field strength). The CQI value “1” indicates the smallest SIR and thus the lowest receive level, and the CQI value “30” indicates the largest SIR and thus the highest receive level.
FIG. 12 illustrates a process flow from the reception of the pilot signal by a mobile terminal to the transfer of data from the base station.
S11: The base station transmits the pilot signal (CPICH: Common Pilot Channel).
S12: The mobile terminal receives the CPICH, then obtains a CQI of its own, and sends the obtained CQI back to the base station. When transmitting the CQI information to the base station, the mobile terminal uses a radio channel called DPCCH (Dedicated Physical Control Channel).
S13: The base station performs the scheduling in accordance with the received CQI.
S14: The base station sends control information obtained as a result of the scheduling, to the selected mobile terminal. When transmitting the control information to the mobile terminal, the base station uses a radio channel called SCCH (Shared Control Channel).
S15: In accordance with the received control information, the mobile terminal switches its transmit/receive function.
S16: Using the communication service set by the scheduling, the base station transmits data to the corresponding mobile terminal. In this case, the data is sent to the mobile terminal via a radio channel called PDSCH (Physical Downlink Shared Channel).
Since the mobile terminal is required to set its transmit/receive function after receiving the control information including the scheduling results, the base station sends the PDSCH after a lapse of a fixed time from the transmission of the control information so that the mobile terminal may be prepared for the reception of data.
FIG. 13 illustrates allocation of the PDSCH to mobile terminals. To allocate the PDSCH to mobile terminals, the base station divides the PDSCH into time slots so that one time slot (2 ms) may be used by a single mobile terminal or shared by two or more mobile terminals.
Also, each cell has a plurality of PDSCH channels, and in the case of HSDPA, there are 15 channels separated in the direction of spreading code (the 15 channels are distinguished from one another by their spreading codes). Thus, in the case of downlink data transmission, the PDSCH is shared in such a manner that the time slots arrayed in both the direction of time and the direction of spreading code are allocated to mobile terminals.
For example, where the time slots t1 of the PDSCH are used to carry data and a certain mobile terminal with high priority is selected by the scheduling, the time slot t1 of the PDSCH channel CH1 only or the time slots t1 of all the 15 PDSCH channels CH1 to CH15 are used to transfer the data to the selected mobile terminal (the scheduling also determines which and how many time slots to use and how many PDSCH channels to use).
In the aforementioned manner, the mobile terminal measures the CQI and transmits the measured CQI to the base station, which then carries out scheduling on the basis of the received CQI so that the scheduling results may be reflected in the data transmission from the base station to the mobile terminal via the PDSCH. The conventional HSDPA communication is, however, associated with the problem that the downlink throughput lowers if the accuracy in the CQI measurement by the mobile terminal is low.
While the mobile terminal remains stationary or is moving at low speeds, variation (dispersion) in the measured CQI value is small, but when the mobile terminal is moving at high speeds, the dispersion of the CQI is significantly great because of channel variation and the like attributable to fading (phenomenon wherein the radio wave receive level fluctuates with movement of the terminal or with time).
FIG. 14 is a conceptual diagram of CQI dispersion characteristic, wherein the horizontal axis indicates the CQI and the vertical axis indicates the frequency (%) of occurrence of identical CQI. It is assumed here that an ideal CQI measured by the mobile terminal in a certain environment is “a” (1≦a≦30).
While the mobile terminal in the environment wherein the CQI is equal to “a” remains stationary or is moving at low speeds, the measured CQI shows a dispersion characteristic curve K1, indicating that the most frequently measured CQI is “a” and that the dispersion has a narrow width with respect to “a”. In contrast, when the mobile terminal is moving at high speeds, the measured CQI shows a dispersion characteristic curve K2, which indicates that the CQI value “a” is measured less frequently and that the dispersion has a greater width with respect to “a”.
Specifically, where the CQI is measured a plurality of times while the mobile terminal remains stationary or is moving at low speeds, an identical CQI value (in this case, “a”) is measured most frequently and other measured values, though not identical with “a”, are close to “a” (the ideal value “a” is measured on most occasions).
On the other hand, where the CQI is measured a plurality of times while the mobile terminal is moving at high speeds, an identical CQI value (“a”) is measured less frequently and also the measured CQI values vary significantly (CQI values greatly different from the ideal value “a” are frequently measured).
Thus, while the mobile terminal remains stationary or is moving at low speeds, the ideal CQI (=a) matching the actual receiving environment is measured on most occasions, and therefore, lowering of the throughput does not occur. Even if CQI values different from the value “a” are measured, the measured CQI values are very close to the ideal value “a”. Accordingly, such slightly different CQI values do not require the base station to greatly change the scheduling results to be reflected in the PDSCH, and a situation where the throughput significantly lowers does not occur.
While the mobile terminal is moving at high speeds, on the other hand, the throughput significantly lowers. Let us suppose that the CQI value “a1”, for example, which is significantly smaller than the ideal value “a” as shown in FIG. 14, is measured and transmitted to the base station for the scheduling.
In this case, even though the actual receiving environment of the mobile terminal satisfies CQI=a and thus is better than the measured environment (“a1”), the base station performs the scheduling on the basis of the measured environment worse than the actual receiving environment of the mobile terminal. As a result, the base station sets a smaller PDSCH format for transferring data, so that the throughput significantly lowers.
Setting a smaller PDSCH format means decreasing the amount of data transferred. For example, in FIG. 13, when CQI=a, data is forwarded by using the time slots t1 and t2 of the ten PDSCH channels CH1 to CH10, and when CQI=a1, data is transferred by using the time slots t1 and t2 of the PDSCH channels CH1 to CH5 or by using only the time slots t1 of the PDSCH channels CH1 to CH10. Consequently, the throughput lowers because the amount of data forwarded from the base station is smaller than that which the mobile terminal can actually receive.
Let us suppose now that while the mobile terminal is moving at high speeds, the CQI value “a2”, for example, which is significantly greater than the ideal value “a” as shown in FIG. 14, is measured and transmitted to the base station for the scheduling.
In this case, although the actual receiving environment of the mobile terminal just fulfills CQI=a and thus is worse than the measured environment (“a2”), the base station performs the scheduling on the basis of the measured environment better than the actual receiving environment of the mobile terminal. Consequently, the base station sets a larger PDSCH format for transferring data, and since retransmission frequently occurs as a result, the throughput significantly lowers.
Setting a larger PDSCH format means increasing the amount of data transferred. For example, in FIG. 13, when CQI=a, data is forwarded by using the time slots t1 and t2 of the ten PDSCH channels CH1 to CH10, and when CQI=a2, data is transferred by using the time slots t1 and t2 of all the 15 PDSCH channels CH1 to CH15 or by using the time slots t1, t2 and t3 of the PDSCH channels CH1 to CH10.
Consequently, the amount of data forwarded from the base station is greater than that which the mobile terminal can actually receive. Since the error rate of the mobile terminal increases, the mobile terminal frequently requests retransmission, with the result that the throughput lowers.
As explained above, the conventional HSDPA communication takes no account of the CQI measurement accuracy of mobile terminals. Accordingly, data transfer control not matching the actual receiving environment is often carried out, giving rise to a problem that the throughput lowers.